Electro-optical measurement of blood characteristics has been found to be useful in many areas of blood constituent diagnostics, such as glucose levels, oxygen saturation, hematocrit, billirubin and others. This method is advantageous in that it can be performed in a non-invasive fashion. In particular, much research has been done on oximetry, a way of measuring oxygen saturation in the blood, as an early indicator of respiratory distress.
Infants during the first year of life are susceptible to breathing disturbances (apnea) and respiratory distress. Sudden Infant Death Syndrome (SIDS) is a medical condition in which an infant enters respiratory distress and stops breathing, leading to the death of the infant. Although the cause and warning signs of SIDS are not clear, it has been shown that early detection of respiratory distress can provide the time to administer the aid necessary to prevent death.
Many types of baby monitors are currently available, from simple motion detectors to complicated systems which stream oxygen enriched air into the infant's environment. Some of the more accepted monitoring methods include chest motion monitors, carbon dioxide level monitors and heart rate (pulse) monitors. Unfortunately these methods often do not give the advance warning necessary for the caregivers to administer aid. In addition, these monitors are administered by attaching a series of straps and cords, which are cumbersome to use and present a strangulation risk.
The chest motion monitor gives no warning when the breathing patterns become irregular or when hyperventilation is occurring, since the chest continues to move. Distress is only noted once the chest motion has ceased at which point there may only be a slight chance of resuscitation without brain damage. In addition these devices are known to have a high level of “false alarms” as they have no way to distinguish between the lapses in breathing which are normal for an infant (up to 20 seconds) and respiratory distress. These devices can cause excessive anxiety for the caregivers or cause them to ignore a signal, which is true after responding repeatedly to false alarms.
Among other symptoms, SIDS causes an irregular heartbeat, resulting eventually in the cessation of heartbeat with the death of the infant. There are some instruments, which use the EKG principle to monitor this clinical phenomenon. This is a limited method that has a very high rate of false positives since the monitors have inadequate algorithms to determine what is a SIDS event. Obviously, this is not a convenient method, nor is it desirable to have the infant constantly hooked up to an EKG monitor.
In light of these disadvantages a better method to use is a form of electro-optical measurement, such as pulse oximetry, which is a well-developed art. This method uses the difference in the absorption properties of oxyhemoglobin and deoxyhemoglobin to measure blood oxygen saturation in arterial blood. The oximeter passes light, usually red and infrared, through the body tissue and uses a photo detector to sense the absorption of light by the tissue. By measuring oxygen levels in the blood, one is able to detect respiratory distress at its onset giving sufficiently early warning to allow aid to be administered as necessary.
Two types of pulse oximetry are known. Until now, the more commonly used type has been transmission oximetry in which two or more wavelengths of light are transmitted through the tissue at a point where blood perfuses the tissue (i.e. a finger or earlobe) and a photo detector senses the absorption of light from the other side of the appendage. The light sources and sensors are mounted in a clip that attaches to the appendage and delivers data by cable to a processor. These clips are uncomfortable to wear for extended periods of time, as they must be tight enough to exclude external light sources. Additionally, the tightness of the clips can cause hematoma. Use of these clips is limited to the extremities where the geometry of the appendages is such that they can accommodate a clip of this type. The clip must be designed specifically for one appendage and cannot be used on a different one. Children are too active to wear these clips and consequently the accuracy of the reading suffers.
In another form of transmission oximetry, the light source and detector are placed on a ribbon, often made of rubber, which is wrapped around the appendage so that the source is on one side and the detector is on the other. This is commonly used with children. In this method error is high because movement can cause the detector to become misaligned with the light source.
It would be preferable to be able to use the other type of pulse oximetry known as reflective, or backscattering, oximetry, in which the light sources and light detector are placed side by side on the same tissue surface. When the light sources and detector can be placed on the tissue surface without necessitating a clip they can be applied to large surfaces such as the head, wrist or foot. In cases such as shock, when the blood is centralized away from the limbs, this is the way meaningful results can be obtained.
One difficulty in reflective oximetry is in adjusting the separation between the light source and the detector such that the desired variable signal component (AC) received is strong, since it is in the alternating current that information is received. The challenge is to separate the shunted, or coupled, signal which is the direct current (DC) signal component representing infiltration of external light from the AC signal bearing the desired information. This DC signal does not provide powerful information. If the DC signal component is not separated completely, when the AC signal is amplified any remaining DC component will be amplified with it, corrupting the results. Separating out the signal components is not a simple matter since the AC signal component is only 0.1% to 1% of the total reflected light received by the detector. Many complicated solutions to this problem have been proposed.
If the light source and detector are moved further apart, this reduces the shunting problem (DC), however, it also weakens the already weak AC signal component. If the light source and detector are moved close together to increase the signal, the shunting (DC) will overpower the desired signal (AC).
Takatani et al., in U.S. Pat. No. 4,867,557, Hirao et al., in U.S. Pat. No. 5,057,695 and Mannheimer, in U.S. Pat. No. 5,524,617 all disclose reflective oximeters that require multiple emitters or detectors in order to better calculate the signal.
A number of attempts have been made to filter out the DC electronically (see Mendelson et al., in U.S. Pat. No. 5,277,181). These methods are very sensitive to changes in signal level. The AC remaining after the filtering often contains a small portion of DC, which upon amplification of the AC becomes amplified as well, resulting in inaccurate readings. Therefore, this method is only useful in cases where the signal is strong and uniform.
Israeli patents 114082 and 114080 disclose a sensor designed to overcome the shunting problem by using optical fibers to filter out the undesired light. This is a complicated and expensive solution to the problem that requires a high level of technical skill to produce. In addition, it is ineffectual when the AC signal is relatively weak.
As can be seen from the above discussion, the prior art methods of addressing the AC/DC signal separation problem in reflective oximetry techniques are complicated and expensive. Therefore, it would be desirable to provide a simple, low cost and effective method for achieving accurate reflective or transmitted oximetry detection of respiratory stress.